Cartridge type electrical fuses having axial leads have been long known in the fuse art. The fuse element in such a fuse is typically a fusible wire supported within a cylindrical open-ended insulating housing for the fuse and closed by metal end caps carrying outwardly axially extending leads. To insure reliable fusing it is essential that the fuse wire must not touch the interior wall of the housing along the portion of its length which can affect its fuse blowing characteristics; hence, the ends of the fuse wire are supported in such a manner as to prevent such contact. In some fuse designs, the fuse element extends diagonally across the sleeve ends. In such case, the lead carrying end caps having solder therein are used to capture the fuse wire ends folded over the outside of the sleeve ends. The final mechanical assembly consists of press fitting the end caps over the foldedover ends of the fuse wire followed by momentary heating of the solder to obtain good electrical connection between the fuse wire and the end caps.
Where the fuse was a miniature fuse having a housing typically made of ceramic material which cannot be solder bonded without adding a metal coating, the only substantial opposition to the separation of the end caps from the sleeve was derived from the pressure fitting of the end caps over the outer surface of the housing. Thus, such fuse structures were generally weak in tension, and were prone to mechanical failure on a pull test applied to the end leads. An alternative construction was to solder bond the end caps to the housing ends, which requires an expensive local outer metallization of the housing ends. Such structures are prone to humidity induced corrosion problems because of the exposed metal end caps and the lack of any hermetic sealing thereof.
One prior art partial solution to the above-mentioned problems was the application of a length of heat-shrinkable plastic tubing tightly heat shrunk over the housing and end caps, the tubing overlapping, although loosely, the inner ends of the leads extending outwardly from the end caps. The heat shrunk tubing provided some improvement in fuse strength and provided a moderately good sealing for the fuse interior. A disadvantage of this construction was that the cap ends are exposed to the external ambient conditions, owing to the fact that the limited shrinkage capability of the tubing prevented a desired end cap sealing engagement of the heat shrunk tubing prevented a desired end cap sealing engagement of the heat shrunk tubing with the leads. Such sealing is desirable when the fuse is used on printed circuit boards which, after complete assembly of parts on the board, is often dropped into a liquid solvent to clean the board. Also to impart a desired adequate corrosion resistance to the end caps, it was still necessary to plate the still exposed end caps with a corrosion resistance material.
In the fuse encased by the shrink fitted tubing, the resulting structure was still not adequately strong, in that a moderate pull on the leads can still sometimes shift the end caps to break the fuse wire. The shrink tube fitted fuse as described also was more costly to manufacture than desired. Also, because the fuse housing was made of an opaque ceramic material, the fuse wire was not visible even when the shrink tubing was transparent.
To partially overcome the problems referred to, the invention disclosed in U.S. Pat. No. 4,460,887 was developed. As disclosed therein, instead of using shrink tubing as described, an encapsulating body of insulating material, such as an opaque epoxy material, was applied to the fuse so as to cover, seal and physically interconnect the exposed exterior surfaces of the housing and the end caps and to at least seal around the leads extending from the end caps.
As explained in this patent, the epoxy insulating coating was formed by initially applying epoxy powder to a rotated fuse which had been preheated to fuse the epoxy powder. Because of the temperatures involved, it was discovered that blow holes sometimes developed in the epoxy encapsulating body because of out-gassing caused by the heating of the fuse to the epoxy powder fusing temperature. When these fuses were subject to severe high current overload circuit tests, it was found that the blow holes undesirably reduced the insulation resistance of the encapsulating body. Also, the epoxy material was a translucent material so that the fuse wire was not visible, even if the fuse housing were to be made of a transparent material.
It is one of the objects of the present invention to provide a new miniature fuse which is less costly, more reliable and/or more effective than that just described.